Effect of high-temperature stress on the activity of key enzymes in the AsA-GSH cycle in ‘Yali’ pears

Abstract

Through treating fruits at higher temperatures, choosing natural fruits on different exposures on a tree canopy and collecting exclusively both sunburn and normal fruits, the metabolic patterns were studied by comparing the activity of key enzymes in the ascorbate-glutathione (AsA-GSH) cycle including ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), and glutathione reductase (GR). The results indicated that the activity of three enzymes increased under around less than 35°C acclimating conditions, with no significant changes in membrane electrolyte leakage. The activity of APX, MDAR, and GR rose during the initial stage when fruits were treated at 45°C but decreased significantly as treating time extended. A time response existed in APX, MDAR, and GR to high temperature. For example, APX activity reached the maximum when fruits were treated for 1 h, however, MDAR and GR showed the peak when fruits were treated for 3 h and 5 h, respectively, implying the possible acting sequence relationship of three enzymes in the AsA-GSH cycle. In view of the whole cycle, APX served as the first enzyme directly scavenging active oxygen species, followed by a series of chain reactions to regenerate ascorbic acid (AsA), and GR served as the last post in the cycle. Because various ecological conditions existed in a tree canopy, there was a significant difference in the enzymatic activity among fruits bearing on either exterior or interior canopy. A significantly higher activity of APX and MDAR was found with exterior fruits, compared with interior ones, which may be regarded as a frequent acclimation to high temperature and excessive sunlight.